1. Field of the Invention
The present invention relates to a scintillator crystal and to a radiation detector.
2. Related Background Art
Conventional radiation detectors that comprise scintillators are known as “scintillator-type radiation detectors”. In such radiation detectors, radiation such as γ-rays first interact on the scintillator and generate fluorescence. The generated fluorescence is then detected by a photodetector such as a photomultiplier tube and converted to an electrical signal. The electrical signal is processed by various electronic circuits to obtain information such as the counting rate, fluorescent light quantity and time data. The obtained information is used to determine the intensity, energy and emitting position and direction of the incident radiation. Such scintillator-type radiation detectors are used in a wide variety of fields including, primarily, nuclear medicine, high energy physics, radiation control and underground stratigraphic inspection.
Scintillator-type radiation detectors generally have inferior energy discretion compared to semiconductor detectors, another known means for γ-ray detection. However, some scintillator-type radiation detectors have higher densities and effective atomic numbers than semiconductor detectors, and their response times for single rays are also shorter. It may therefore be concluded that scintillator-type radiation detectors are more suitable than semiconductor detectors for detection of high energy γ-rays, or when highly precise time data or high detection efficiency are required.
Some of the characteristics required for scintillator-type radiation detectors for γ-ray detection include high γ-ray detection efficiency, excellent energy discretion and superior time resolution. Among the characteristics required for scintillators that exhibit all of these, there may be mentioned high density and effective atomic number, large fluorescent light quantity (fluorescence intensity), a fluorescent wavelength suitable for the wavelength sensitivity of the photodetector, excellent energy resolution and a rapid fluorescence rise and short decay time.
A higher density and effective atomic number of the scintillator increases the probability of interaction between the γ-rays and scintillator and can thus increase the detection efficiency. A short fluorescent decay time results in a shorter processing time for each γ-ray thus allowing signal processing for more γ-rays in a shorter time, for a higher “time sensitivity”. On the other hand, in order to increase the energy discretion of the scintillator-type radiation detector it is necessary to minimize quantum conversion in the corresponding photodetector and fluctuation during amplification in the circuit that processes the obtained electrical signal. This can only be realized with a large fluorescent light quantity (fluorescence intensity) at the fluorescent wavelength suited for the sensitivity wavelength of the photodetector.
Moreover, as mentioned above, superior energy resolution of the scintillator itself is also important. When two γ-rays interact essentially simultaneously on the scintillator, fluorescence emissions are produced from the scintillator for each γ-ray. The time resolution of the scintillator-type radiation detector may be evaluated, for example, based on distribution when statistically measuring the time variance in the electrical signal obtained from these fluorescence emissions. Superior time resolution can only be achieved by enlarging the electrical signal at the small time Δt from the instant fluorescence is produced. The conditions for the scintillator to enlarge the electrical signal include a large fluorescent light quantity, a fluorescent wavelength suited for the photodetector, a rapid rise of fluorescence and a short decay time.
A Positron Emission Tomography (PET) device in a nuclear medicine diagnostic apparatus, typical of those widely used in recent years, will now be explained as an example of application of a scintillator-type radiation detector. A PET apparatus is a device that images the distribution of a drug composed mainly of a sugar, for example and containing a positron-emitting nuclide-containing species, that has been administered to the body of a subject. This type of PET apparatus can be used to detect even initial-stage cancerous masses with millimeter unit sizes.
The positrons emitted from the drug immediately bond with neighboring electrons to cause pair annihilation, whereupon a pair of annihilation γ-rays are emitted in directions 180° to each other. The γ-rays are captured simultaneously by a plurality of scintillator-type radiation detectors arranged in ring. Since the drug lies on a straight line connecting the two scintillators onto which the γ-rays are interacting, it is possible to acquire a drug distribution in the body by reconstructing an image from information based on the γ-rays.
Since the resolution of the image is of utmost importance in a PET apparatus, the size of each scintillator element must be reduced. It is preferred for diagnosis with a PET apparatus to be completed within a short period of time. In order to achieve this, it is desirable for the PET apparatus to have improved detection efficiency for annihilation γ-rays with a high energy dose of 511 keV.
In order to distinguish between scattering of annihilation γ-rays that occurs in the body and external γ-rays, it is important for the scintillator-type radiation detector to have high energy discretion. In addition, since a very brief time window exists for coincidence of the pair of annihilation γ-rays, it is preferred for the scintillator-type radiation detector to have excellent time resolution.
Scintillators are largely classified as either organic scintillators or inorganic scintillators, based on their structural materials. As inorganic scintillators there may be mentioned those employing materials such as NaI:Tl, CsI:Tl, Bi4Ge3O12 (BGO), Gd2SiO5:Ce (GSO) (for example, see Japanese Patent Application Laid-Open SHO No. 62-008472 and Japanese Patent Application Laid-Open No. 2003-300795), Lu2SiO5:Ce (LSO) (for example, see Japanese Patent Publication No. 2852944) and the like.
Most known among these are inorganic scintillators employing NaI:Tl as the material. Such inorganic scintillators have become the most commonly and almost exclusively used type of scintillators in the field of γ-ray detectors, since their discovery by R. Hofstadter in 1948 until the present time. Because NaI is hygroscopic it must be subjected to waterproof treatment with packaging, for example, when it is used. Such inorganic scintillators, however, have excellent cost performance and are appropriate for large crystal sizes, while their fluorescent light quantities are large and their fluorescent wavelengths are suitable for reading by photomultiplier tubes. The drawbacks of inorganic scintillators include rather low density, slow fluorescence rise and long fluorescent decay time.
Other inorganic scintillators, such as those employing CsI:Tl, have weaker deliquescence and large fluorescent light quantity compared to those employing NaI:Tl. On the other hand, the fluorescence rise time and decay time is longer than NaI:Tl, while the density is not very high. Scintillators employing BGO have very large densities and effective atomic numbers, as well as very strong deliquescence. However, such scintillators have drawbacks including low fluorescent light quantity, fluorescent wavelengths that are not suited for photomultiplier tubes, and long decay times.
Scintillators employing GSO:Ce were the first to utilize the high luminous efficiency and rapid decay of Ce. Such scintillators have rapid decay times and excellent energy resolution. However, their fluorescent light quantity is not very large and the fluorescence rise times are slow. LSO:Ce scintillators have short fluorescence rise times and decay times, with large fluorescent light quantity and fluorescent wavelengths suitable for photomultiplier tubes. These scintillators, however, are limited in their uses because of their unsatisfactory energy resolution and their very high autofluorescence due to radioactive isotopes in Lu.
For this reason, rare earth halide single crystals and rare earth alkali halide single crystals, comprising Ce as the activator are becoming a focus of interest as new scintillator materials. Previously disclosed rare earth alkali halides for use in scintillators include RbGd2Br7:Ce (for example, Nuclear Instrument And Methods In Physics Research A486 (2002) p. 208, and Japanese Patent Application Laid-Open No. 2006-508227), K2LaCl5 (for example, Nuclear Instrument And Methods In Physics Research A486 (2002) p. 208). Previously disclosed rare earth halides for use in scintillators include LaCl3 (for example, Japanese Patent Application Laid-Open No. 2004-500462), LaBr3 (for example, Japanese Patent Application Laid-Open No. 2003-523446 and Nuclear Instruments and Methods In Physics Research A486 (2002) p. 254), CeBr3 (for example, IEEE Transactions Nuclear Science, Vol. 52 (2005) p. 3157), LuI3:Ce (for example, Nuclear Instrument And Methods In Physics Research A537 (2005) p. 279), LaCl3xBr3(1-x):Ce (for example, Japanese Patent Application Laid-Open No. 2005-120378) and CeCl3xBr3(1-x) (for example, Japanese Patent Application Laid-Open No. 2006-241458). In particular, scintillators employing such rare earth halide single crystals offer the advantage of large fluorescent light quantity, excellent energy resolution and short fluorescent decay time.
Among these, LaBr3:Ce and CeBr3 have superior density, effective atomic number, fluorescent light quantity, energy resolution, fluorescence rise and fluorescent decay time for the scintillator employing the material, compared to NaI:Tl. These materials are therefore expected to become widely used in fields of scintillator-type radiation detector applications. In particular, the materials are noted for their high energy resolution and high time resolution for radiation detectors incorporating the scintillators. Scintillators employing such materials, therefore, are considered very promising for TOF-PET applications that incorporate TOF (Time of Flight) systems, which are candidates for second generation PET in nuclear medicine (for example, see International Publication Number WO 04/044613 pamphlet).
The major characteristics of inorganic scintillators employing conventional materials as explained above are listed in Table 1.
TABLE 1FluorescentFluorescentFluorescentlightpeakdecayDensityquantitywavelengthtimeMaterial(g/cm3)(photon/MeV)(nm)(ns)NaI:TI3.6738000415230CsI:TI4.53520005401000Bi4Ge3O127.138000480300Gd2SiO5:Ce6.711000044030-60Lu2SiO5:Ce7.42600042042LaBr3:Ce5.2961000360, 38015-25CeBr35.268000370, 39017